1. Technical Field
This disclosure generally relates to circuit boards, and more specifically relates to printed circuit boards for tight-pitch components.
2. Background Art
Printed circuit boards are commonly constructed of glass cloth impregnated with various epoxy resins. The glass cloth is typically constructed of bundles of glass fibers woven together in an X-Y orthogonal fashion, with the bundles being perpendicular to each other. The epoxy/glass substrate is laminated to copper foil, forming “cores” which are subsequently etched to form the desired features. These cores are then laminated together using partially cured epoxy/glass layers with temperature and pressure, causing the resin to flow between layers to form a robust composite structure.
One mechanism that may impact reliability in traditional epoxy/glass printed circuit boards is known as Conductive Anodic Filament (CAF) growth. CAF growth results when a pathway exists along or within the individual glass fibers within the printed circuit board where a conductive path may form. In essence, a conductive filament of copper salts may grow along one or more of the glass fibers when there is both voltage and moisture present, such as when equipment operates in a high humidity environment. The conductive filament may eventually grow to a length sufficient to short together two features that should not be connected, such as adjacent plated-through holes in the printed circuit board, causing the printed circuit board to fail. CAF growth has been attributed to one or more of several possible scenarios, including: 1) hollow glass fibers as a result of bubbles in the liquid glass during the extrusion process that forms the glass fibers; 2) insufficient binding of the resin to the glass fibers, which may occur when the resin is initially bound to the glass fibers, or may occur only after exposure to thermal cycles, such as those that occur during normal system operation; 3) insufficient impregnation of the resin into the bundles of glass fibers, leaving small open tracks where multiple glass fibers meet (sometimes called a “triple point”). Note that CAF growth is well-understood in the art, and its underlying mechanisms therefore are not discussed further here.
CAF growth has been documented and understood for some time, but in the past CAF growth has not been a significant reliability concern for printed circuit boards because the time required for a conductive filament to grow between features on a printed circuit board typically has been greater than the life expectancy of the system in which the printed circuit board is located. However, the spacing between conductive leads or contacts on electronic components (referred to in the art as “pitch”) continues to shrink. As a result, the time it takes for a CAF growth to bridge two features on a printed circuit board shrinks correspondingly. With the advent of sub-1 mm and lower pitch components, the CAF growth has now been shown to occur within the time frame of system life expectations. As a result, CAF growth becomes a reliability concern for printed circuit boards with tight-pitch components. One possible way to diminish the possibility of CAF growth is to address the three causes enumerated above during the manufacture of the epoxy/glass printed circuit board. However, these causes of CAF growth are not easily eliminated. Without a way to provide a printed circuit board that has enhanced resistance to failure caused by CAF growth, the electronics industry will be plagued by failures in printed circuit boards that have tight-pitch components due to CAF growth.